The need to bundle items together for easier handling and carrying is centuries old. Until recently, ropes and cords tied around the items were the method of choice. These worked well, but suffered from several drawbacks. While fairly easy to fasten, by tying, the knots typically tighten under load and are difficult to release. Further, because the ropes and cords have only limited elasticity, if the bundle shifts or compresses, the rope may loosen. These, and other, drawbacks have motivated a continued search for a better strap for tying items into bundles.
More recent approaches include the use of elastic cord, often called shock cord or bungee cord. While solving the problem of the lack of elasticity in conventional rope, elastic cord is difficult to fasten. Because of its elastic nature, knots tend to either work loose, or bind, depending on the nature of the cord and the knot used. To solve this, a variety of fasteners have been used. The most common is a simple hook affixed to each end of the cord or a hook through which the cord passes, allowing adjustment of the length of the cord. These solve the fastening problem, but introduce a new problem. The hooks tend to snag on soft items, such as textiles, and to scratch or abrade hard items. This is especially true of the fixed hooks which are often made of formed metal wire. The end of the wire is often exposed, presenting a sharp edge.
Various forms of adjustable tie down straps have also been used. These typically have a hook at each end and an adjustment/tensioning device in the middle. The adjustment device may be a simple cam lock or a ratchet device. These are well suited to restraining heavy loads, especially when hauled by truck or tractor-trailer, but are generally not suited to tying items into bundles. These straps also pose a risk of abrasion or scratching by the hooks and adjustment mechanisms.
Any of the approaches which utilize metal components also pose a risk of sparking. While often not a concern, these straps are not suitable for those (circumstances where the environment may contain high levels of combustibles. This would be true of many petroleum product or chemical handling environments. Unfortunately, elastic materials are difficult to couple without some type of hardware component. While plastic i s usable, it is often bulkier or weaker than metal.
Several US patents have been issued on straps and ties which do not use additional hardware components for fastening. U.S. Pat. No. 3,913,179 to Rhee is typical of such. It discloses a tie strap made of a flexible material, such as plastic, which uses slots in the strap and a barbed head to provide a connection. While solving some of the above problems, it is not a complete solution. The strap is easily fastened because the head can be inserted at an angle into the T-slot and then rotated into position. However, this approach would not work if an elastic material were to be substituted for the preferred plastic. The slots are too large relative to the head of the strap and would easily deform to allow the head to be withdrawn. The barb uses 90 degree ledges at the rear which work well with a rigid retaining slot but which provide insufficient grip for use with elastic material. Because the material can not be truly elastic with this approach, the straps still exhibit the problem of loosening if the bundle shifts or compresses. Additionally, if the load limit of the strap is exceeded, there is little or no indication prior to a sudden release of the coupling or snapping of the strap itself.
U.S. Pat. No. 5,581,850 to Acker is similar in many respects to the Rhee patent. A flexible, but not elastic material is used, with a barbed head passing through an elongated slot in the strap. As in Rhee, the barb uses right angle ledges. The barb profile and the size of the slots combine in a design which will not work with an elastic material. The slot would easily deform to allow withdrawal of the head.
U.S. Pat. No. 5,012,558 to Willoughby does present a design utilizing an elastic material. Here too, the barb uses a ledge which is at right angles, or even less. A retaining hole which is very small relative to the size of the head is used to avoid the above problems with deformation. Further, the strap is designed to pass the head through multiple holes in order to achieve the desired retention force. Unfortunately, this design requires significant force to insert the head of the strap into each of the holes. Withdrawing the strap from the holes when the user desires to release the strap requires greater force because they are working against the barb. Generally the same amount of force will be required to release the strap intentionally as it supplies for holding items together.
There is a need for a tie strap which can be made from an elastic material and which can be reliably connected to itself, or another strap, without the use of additional hardware components. It should be easy to make the connection and to release the connection when desired. However, it should be very difficult for the strap to release on its own when in use within proper load limits. Preferably, the strap would be compact and easily stored. Ideally, the strap will require significantly less force to release it than it can resist when holding a load. Ideally, the strap would provide a visual indication as load limits are approached and would release smoothly, rather than breaking, when the load limit is exceeded. Also ideally, the strap would be made from a single material and would require no bonding or other connections, in order to reduce manufacturing complexity and cost.